(Takes place prior to "The Big Mac Question" for obvious reasons.)
Big Macintosh's romantic relationship with Sugar Belle is hardly a secret, everypony in town knows about it. Even Cheerilee.
In fact, it's because of the relationship that Cheerilee has arranged to meet with Sugar Belle after the end of classes one day. The meeting is something that Sugar Belle is understandably nervous about since she knows about Cheerilee and Big Mac dating.
But Sugar Belle doesn't know why Cheerilee has called her in, it isn't for the reasons the up and coming unicorn baker might think. Cheerilee has something she wants Big Mac's girlfriend to understand and something she wants to make perfectly clear about her own feelings for the stallion.